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SAN DIEGO (AP) — A blackout that swept across parts of the Southwest and Mexico apparently began with a single utility worker and a minor repair job.

How it then rippled from that worker in the Arizona desert, to southern California and across the border, plunging millions of people into darkness, has authorities and experts puzzled, especially since the power grid is built to withstand such mishaps.

However it spread, Thursday’s outage was a reminder that the nation’s transmission lines remain all too vulnerable to cascading power failures.

“There are a lot of critical pieces of equipment on the system and we have less defense than we think,” said Rich Sedano at the Regulatory Assistance Project, a utility industry think tank based in Montpelier, Vt.

There have been several similar failures in recent years. In 2003, a blackout knocked out power to 50 million people in the Midwest and the Northeast. And in 2005, a major outage struck the Los Angeles metropolitan area.

That same year, Congress required utilities to comply with federal reliability standards for the electricity grid, instead of self-regulation. Layers of safeguards and backups were intended to isolate problems and make sure the power keeps flowing.

But that didn’t happen on Thursday.

The Arizona Public Service Co. worker was switching out a capacitor, which controls voltage levels, outside Yuma, Ariz., near the California border. Shortly after, a section of a major regional power line failed, eventually spreading trouble further down in California and later Mexico, officials said.

And the lights began to go out in a border region of roughly 6 million people.

The outage knocked out traffic lights, causing gridlock on the roads in the San Diego area. Two reactors at a nuclear power plant up the California coast went offline after losing electricity. More than 2 million gallons of sewage spilled into the water off San Diego, closing beaches in the nation’s eighth-largest city.

Many had to spend the night, on both sides of the U.S-Mexico border, struggling to fall asleep in the high temperatures.

Federal and state investigators are trying to determine what caused the blackout and how future problems can be prevented. If regulatory violations are found, the government could issue fines of up to $1 million per day for every violation, officials said.

Among the questions they will be asking is why the safeguards to keep power flowing appeared to work, at least at first. There was a roughly 10-minute gap between the time the power line failed and customers lost electricity, said Daniel Froetscher, vice president of energy delivery for Phoenix-based APS.

The line has been “solid, reliable” with no history of problems, Froetscher said.

San Diego Gas & Electric Co. should have isolated the problem by shutting down the 500-kilovolt Southwest Powerlink as it did during 2007 wildfires, said Michael Shames, executive director of the advocacy group, Utility Consumers’ Action Network.

“If a fire breaks out in the kitchen, the first thing you do is shut the door to the kitchen to stop it from spreading,” he said.

He also questioned why the San Onofre nuclear power plant was forced to shut down, and why other back-up energy didn’t kick in.

Shames said blaming the Arizona utility worker would be like overlooking the role of wooden buildings and inadequate firefighting protection in Chicago’s 1871 fire.

“It’s sort of like saying the main reason for the Great Chicago Fire was the cow. The cow started the fire by kicking over the lantern but that’s not what caused it,” he said.

Michael Niggli, SDG&E’s president and chief operating officer, said the company had no time to shut down the line because it had no warning.

Niggli said automatic circuit-breakers at San Onofre prevented the blackout from spreading to Southern California Edison, which serves 14 million people in the Los Angeles area.

At a news conference Friday, Niggli compared the power grid to a quiet pond. “When somebody throws a rock in there, it causes ripples. Depending on how big that rock is, those ripples are going to affect everyone that’s in that pond,” he said.

Experts say the problem could have been made worse by the way power flows into California.

California imports huge amounts of power from Arizona and other states. When the voltage fluctuations caused the San Onofre nuclear station to shut down to protect itself, it deprived the grid of a huge source of California-generated power.

Normally, a loss of that power would result in more flowing from Arizona.

But that power was already off line, depriving the region of power.

In some ways, the nation’s power grid is not very different than it was when it was first being laid out more than a century ago. It takes power from generating stations, adapts it for use at homes and businesses and moves it where it is needed.

The grid is also extraordinarily complex because of the nature and speed of the flow of electricity.

Electricity, like water, flows in the direction of least resistance. If one path is blocked, lots of electricity can suddenly flow to a line or piece of equipment that is designed to handle only a trickle.

Electricity flows extremely fast. A problem in one location can impact a piece of equipment hundreds of miles away almost instantaneously. It’s so complex that planners and engineers can’t accurately model how it will behave, especially when things go wrong.

Utilities, grid operators and regulators struggle with how much protection to build into the system without over-spending and cutting into profits, raise customer rates too high, or both.

A capacitor failure could cause voltage fluctuations that could destabilize a system, but experts say a single capacitor outage in normal circumstances should never trigger an outage as massive as Thursday’s, which was echoed by the Arizona company.

“These things happen every day and no one notices them because the system responds. In this case, it didn’t,” said Vikram Budhraja, president of Electric Power Group, a consulting firm based in Pasadena, Calif.